Nemesis and Slave are part of the upgrade path into Austere (together with Invade and Ares), so they won't be useless, plus I'm also expecting them to remain the strongest weapons until mid Ep4.
There's no way 15 stars weapons will come with Ep4, 14 stars sure, eventually, but definitely not 15 stars. I would expect 15 stars during Ep5.
Equipment releases, I'm expecting something like this to happen going forward:
- The next major urgent quest will introduce Invade weapons and the option to upgrade Ray weapons and units into Union.
- Challenge quests, maybe they'll convert the Ideal series into NT weapons for NA and buff them up to make them compete with Nemesis/Slave?
- Mining Base 4, maybe they'll convert the Orbit series into NT weapons for NA?
- The first Ep4 urgent quest will add the Earth series weapons (like Seikaiser Sword), while they're considerably weaker than Nemesis/Slave, they come with Stat VI augment factors (like Might VI), which makes them very useful for augmenting.
- And Ultimate Amduscia will likely introduce the first 14 star weapons, Ares series.
When it comes to Zieg, I guess at first he'll only have the option to upgrade Ray into Union, Invade units into Austere units, and later on by the end of Ep4, Zeinesis into Qliphad.
It's also possible there could be some exchanges introduced earlier than JP (Ceres -> Ceres Zara or Homura -> Shien/Guren), or even exchanges that don't exist on JP (Nox -> Arion -> Raete, the latter 2 are from PSO2es, they're 13 star recolors of Nox, it would be interesting to have a Nox upgrade path on NA, another one would be Revolsio -> Dominacio).